In many geodetic applications, methods and systems for position determination of a geodetic instrument are used, which methods and systems are based on the propagation of signals which can be shadowed and thus limited in their usability. Global positioning systems, such as, for example, GPS, GLONASS or the European Galileo system under construction are an example of such position-determining systems. These systems are based on the reception of satellite signals in a form as undisturbed as possible. In the immediate vicinity of obstacles, the reception of the signal may be limited or completely impossible owing to the shadowing effect thereof, so that a position determination is no longer possible by means of the system. A further example is the position determination of a reflector-bearing instrument comprising a theodolite or tacheometer. By means of a direction and distance measurement with the tacheometer to the geodetic instrument the position of the instrument can also be determined when the position of the tacheometer is known. A precondition for the measurement here is the visual link between the two components. If this link is interrupted, for example by growth or buildings in the region of sight, the position determination method fails.
Methods which are based on a determination of the position of the instrument itself relative to objects whose position is known are known for a position determination of the actual position, i.e. of the current location of the instrument, also in a dead range shadowed in such a manner. An example of this is the classical trilinear surveying method.
In many cases, a geodetic instrument has only a capability for distance measurement, or a measurement of angles cannot be carried out with the required precision or speed. In these cases, the position determination must be carried out by distance measurements alone. For this purpose, the distances to a plurality of points having a known position are measured, and the determination of the actual position can be effected by known methods, as also used, for example, in photogrammetry; correlation methods or correlation calculations are an example of this. The number of points required is dependent on the position thereof and the intended accuracy of the measurement. However, apart from particularly advantageous configurations, as a rule at least 3 or 4 points are required. If an angle is additionally taken into account, for example by additionally measuring the angle relative to the horizontal, the number of points can be reduced.
In order also to be able to use positions in the dead range for surveying by a purely distance-measuring method, it is first necessary to survey, from known locations, the reference points subsequently required for referencing.
A surveying device suitable for this purpose is described, for example, in European Patent EP 0 403 585 B1. The surveying device has a receiver for a satellite position-measuring system and a preferably electrooptical telemeter or telemeter based on the ultrasonic principle. Both components are mounted on a plumbing staff which can be precisely positioned with its plumbing staff tip and has an inclinometer and a display of the vertical alignment. Optionally, a sensor reacting to the earth's magnetic field may also be present. By means of at least 2 measurements to a point from 2 different known positions which are determined, for example, by the satellite position-measuring system the position of this point can now be determined even if it is within the dead range.
Conversely, the position of such a surveying device in the dead range can also be determined by measurements to a plurality of known points in the trilinear surveying method.
If surveying positions for the geodetic instrument which are both in spaces detected by the positioning system and in dead ranges are to be used in a survey, it is necessary, before using the dead range, to survey corresponding reference points for a subsequent determination of the actual position in the dead range. If it is intended to dispense with an explicit angle measurement, as a rule the distances must be measured at least for three to four points from at least three to four known positions, in order to permit an unambiguous position determination. The number of points actually required in each case is dependent on the position of the known points and any possible limitations for reducing ambiguity. In the case of three distance measurements to a point, a plane across which the position to be determined can be reflected is defined by the three known points. The resulting solution comprises two possible positions, of which, however, one position can generally be ruled out for plausibility reasons, for example because it would lie below the earth's surface, or because of simple further information, such as, for example, the distinction between north and south, which can also be made by means of a simple magnetic compass. An unambiguous determination with three known points is possible if advantageous geometrical conditions are present. This is the case, for example, if the position sought lies on a connecting line between two known points.
Depending on the circumstances of a measurement, at least between 9 and 16 distance measurements must therefore be carried out, for which in each case a correct assignment of measurement to reference point and known position must be effected. Carrying out such measurements manually therefore means a high handling effort and error-producing complexity.
Moreover, in order to achieve sufficient accuracy of the position determination, reference points suitable because of their geometrical arrangement have to be chosen. The position change between the known positions which is to be consciously implemented also has an adverse effect on the measuring process. Finally, the extension of the regions detected by the positioning system and shadowed must be actively observed with such methods of the prior art in order to be able to carry out in good time a change of the positioning method to be used.